U.S. patent application number 11/481822 was filed with the patent office on 2007-01-18 for image forming apparatus capable of a color image, and image forming method for forming a color image.
Invention is credited to Youichiroh Miyaguchi, Katsuo Sakai, Tomoko Takahashi.
Application Number | 20070013924 11/481822 |
Document ID | / |
Family ID | 37661376 |
Filed Date | 2007-01-18 |
United States Patent
Application |
20070013924 |
Kind Code |
A1 |
Sakai; Katsuo ; et
al. |
January 18, 2007 |
Image forming apparatus capable of a color image, and image forming
method for forming a color image
Abstract
An image forming apparatus for forming a color image, including
a charging device configured to charge a moving image bearing
member; a writing device configured to form a latent image the
image bearing member by exposing a plurality of colors according to
image information; and a developing device configured to develop
the latent image with toner, wherein the writing device and the
developing device are configured to perform exposure and
development more than once to the image bearing member, which is
charged once by the charging device so that the color image is
formed from superimposed toner of each of the plurality of colors
on the image bearing member.
Inventors: |
Sakai; Katsuo;
(Yokohama-shi, JP) ; Takahashi; Tomoko;
(Yokohama-shi, JP) ; Miyaguchi; Youichiroh;
(Yokohama-shi, JP) |
Correspondence
Address: |
C. IRVIN MCCLELLAND;OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Family ID: |
37661376 |
Appl. No.: |
11/481822 |
Filed: |
July 7, 2006 |
Current U.S.
Class: |
358/1.4 ;
399/40 |
Current CPC
Class: |
G03G 2215/0653 20130101;
G03G 15/0152 20130101; G03G 15/0194 20130101 |
Class at
Publication: |
358/001.4 ;
399/040 |
International
Class: |
G06K 15/22 20060101
G06K015/22 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 11, 2005 |
JP |
2005-202067 |
Claims
1. An image forming apparatus for forming a color image,
comprising: a charging device configured to charge a moving image
bearing member; a writing device configured to form a latent image
on the image bearing member by exposing a plurality of colors
according to image information; and a developing device configured
to develop the latent image with toner of each of the plurality of
colors, wherein the writing device and the developing device are
configured to perform exposure and development for each color of
the plurality of colors after a single charge of the image bearing
member by the charging device, and wherein, for each color, the
developing device is configured to superimpose the toner of the
color on the image bearing member after exposure for the color is
performed by the writing device, so that the color image is formed
from superimposed toner of each of the plurality of colors on the
image bearing member.
2. The image forming apparatus of claim 1, wherein the developing
device is configured to develop the latest image by making the
toner hopping with a phase-shifted electric field.
3. The image forming apparatus of claim 1, wherein the charging
device is a contact-type charging device.
4. The image forming apparatus of claim 3, wherein the contact-type
charging device is a charge-injection-type charging device.
5. The image forming apparatus of claim 1, wherein the exposing
device is configured to expose an image portion forming pixel
supposed to be adhered with toner at a next development so as to
decrease an electric potential of the image portion forming pixel
in absolute value to a lower level than an electric potential of a
blank portion forming pixel that is not exposed.
6. The image forming apparatus of claim 5, wherein the exposing
device is configured to expose to a blank portion forming pixel
that has a higher electric potential in absolute value than another
blank portion forming pixel of blank portion pixels that are formed
in a second or later exposure at a same time as exposure to an
image portion is performed, so that an electric portion of the
blank portion forming pixel becomes a same level as an electric
potential of the another blank portion.
7. The image forming apparatus of claim 5, wherein the exposing
device is configured to expose to a blank portion forming pixel
that has a higher electric potential in absolute value than another
blank portion forming pixel of blank portion pixels that are formed
in a second or later exposure before or after exposure to an image
portion is performed so that an electric portion of the blank
portion forming pixel becomes a same level as an electric potential
of the another blank portion.
8. The image forming apparatus of claim 1, wherein the exposing
device is configured to expose a blank portion forming pixel
supposed not to be adhered with toner at a next development so as
to decrease an electric potential of the image portion forming
pixel in absolute value to a lower level than an electric potential
of a blank portion forming pixel that is not exposed.
9. The image forming apparatus of claim 8, wherein the exposing
device is configured to expose to an image portion forming pixel
that has a higher electric potential in absolute value than another
image portion forming pixel of image portion pixels that are formed
in a second or later exposure at a same time as exposure to a blank
portion is performed, so that electric portion of the image portion
forming pixel becomes a same level as an electric potential of the
another image portion.
10. The image forming apparatus of claim 8, wherein the exposing
device is configured to expose to an image portion forming pixel
that has a higher electric potential in absolute value than another
image portion forming pixel of image portion pixels that are formed
in a second or later exposure before or after exposure to a blank
portion is performed, so that an electric portion of the image
portion forming pixel becomes a same level as an electric potential
of the another image portion.
11. The image forming apparatus of claim 1, wherein the charging
device is configured to charge so that an electric potential
becomes more than a maximum electric potential of a layer of toner
that is developed on the image bearing member.
12. An image forming apparatus for forming a color image,
comprising: means for charging a moving image bearing member; means
for writing to the image bearing member to form a latent image on
the image bearing member by exposing a plurality of colors
according to image information; and means for developing the latent
image with toner of each of the plurality of colors, wherein the
means for writing and means for developing perform exposure and
development for each color of the plurality of colors after a
single charge of the image bearing member by the means for
charging, and wherein, for each color, the means for developing
superimposes the toner of the color on the image bearing member
after exposure for the color is performed by the means for writing,
so that the color image is formed from superimposed toner of each
of the plurality of colors on the image bearing member.
13. The image forming apparatus of claim 12, wherein the exposing
means is configured to expose an image portion forming pixel
supposed to be adhered with toner at a next development so as to
decrease an electric potential of the image portion forming pixel
in absolute value to a lower level than an electric potential of a
blank portion forming pixel that is not exposed, and to expose to a
blank portion forming pixel that has a higher electric potential in
absolute value than another blank portion forming pixel of blank
portion pixels that are formed in a second or later exposure at a
same time as exposure to the image portion is performed, so that an
electric portion of the blank portion forming pixel becomes a same
level as an electric potential of the another blank portion.
14. The image forming apparatus of claim 12, wherein the exposing
means is configured to expose an image portion forming pixel
supposed to be adhered with toner at a next development so as to
decrease an electric potential of the image portion forming pixel
in absolute value to a lower level than an electric potential of a
blank portion forming pixel that is not exposed, and to expose to a
blank portion forming pixel that has a higher electric potential in
absolute value than another blank portion forming pixel of blank
portion pixels that are formed in a second or later exposure at a
same time as exposure to the image portion is performed, so that an
electric portion of the blank portion forming pixel becomes a same
level as an electric potential of the another blank portion.
15. The image forming apparatus of claim 12, wherein the exposing
means is configured to expose a blank portion forming pixel
supposed not to be adhered with toner at a next development so as
to decrease an electric potential of the image portion forming
pixel in absolute value to a lower level than an electric potential
of a blank portion forming pixel that is not exposed, and to expose
to an image portion forming pixel that has a higher electric
potential in absolute value than another image portion forming
pixel of image portion pixels that are formed in a second or later
exposure at a same time as exposure to the blank portion is
performed, so that an electric portion of the image portion forming
pixel becomes a same level as an electric potential of the another
image portion.
16. The image forming apparatus of claim 12, wherein the exposing
means is configured to expose a blank portion forming pixel
supposed not to be adhered with toner at a next development so as
to decrease an electric potential of the image portion forming
pixel in absolute value to a lower level than an electric potential
of a blank portion forming pixel that is not exposed, and to expose
to an image portion forming pixel that has a higher electric
potential in absolute value than another image portion forming
pixel of image portion pixels that are formed in a second or later
exposure before or after exposure to the blank portion is
performed, so that an electric portion of the image portion forming
pixel becomes a same level as an electric potential of the another
image portion.
17. The image forming apparatus of claim 12, wherein the charging
means is configured to charge in order that an electric potential
becomes more than a maximum electric potential of a layer of toner
that is developed on the image bearing member.
18. A method for forming a color image, comprising: charging a
moving image bearing member once; writing to the image bearing
member so as to form a latent image by exposing a color of a
plurality of colors according to image information; developing the
latent image with toner of the color by superimposing the toner of
the exposed color on the latent image; and repeating the writing
and developing steps for each of the plurality of colors so as to
form the color image from superimposed toner images of each of the
plurality of colors on the image bearing member.
19. The image forming method of claim 18, wherein a charged
electric potential is more than a maximum electric potential of the
layer of toner that is developed on the image bearing member.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is based on and claims priority to
Japanese Patent Application No. 2005-202,067 filed Jul. 11, 2005,
the entire contents of which are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates in general to an image forming
apparatus configured to form a full-color image by superimposed
colors with a one-time charge of an image bearing member.
[0004] 2. Discussion of the Background
[0005] An image forming apparatus, such as a printer, a facsimile
machine, a copy machine, a plotter, or a printer/facsimile/copy
complex machine, is known to form an image according to the
following electrophotographic process: charging an image bearing
member (also referred to herein as a "photo conductor"), forming an
electrostatic latent image, developing the image by providing
adherence of powder (also referred to herein as "toner particles"),
to create a toner image, and transferring the toner image to other
medium.
[0006] Currently in an image forming apparatus using the above
electrophotographic process, there are two different processes in a
full-color image forming apparatus that superimposes color on
color. One type is to rotate one image bearing member four times.
During each rotation, a process of uniform electrostatic charge, a
process of exposing the image, a process of forming an image by
development with any one of color toners (Cyan, Magenta, Yellow,
Black), and a process of transferring the developed image to an
intermediate transfer member or recording medium so as to fit
position each other, are performed. In the other type of process,
four photo conductors are laterally arranged. For each photo
conductor, the process of uniform electrostatic charge, the process
of exposure image, the process of forming an image by development
with any one of color toners (Cyan, Magenta, Yellow, Black), and
the process of transferring the developed image on each photo
conductor to an intermediate transfer member or a recording medium
so as to fit position each other is performed.
[0007] However, the one photo conductor/four rotations type process
has a problem of slow printing speed. And forming an image with the
laterally arranged four photo conductor type process (tandem type
process) has disadvantages of larger and complicated structure, and
higher cost.
[0008] Consequently, another type of process has been designed.
This type is capable of superimposing color toner on other color
toner during one-rotation of one photo conductor (hereinafter
referred to as a "one-photo-conductor/one-rotation-superimpose-type
process"). And, although, there is a method of superimposing
different color toner on a surface of the photo conductor by
one-photo-conductor/four-rotations-type process also, this type of
process has a problem of a slow printing rate. To distinguish
herein the type of rotating one photo conductor four times and
transferring every single color toner image per one-rotation, and
the type of superimposing multiple color toners on the photo
conductor without transferring every single toner image, the former
is referred to as one-photo-conductor/four-rotations/transfer-type
process and the latter is referred to as
one-conductor/four-rotations/superimpose-type process.
[0009] In the above
one-photo-conductor/one-rotation-superimpose-type process, by way
of example, four sets of devices are arranged on the side of a
belt-shaped or drum-shaped photo conductor. Each set forms a toner
image on the photo conductor respectively, Cyan, Magenta, Yellow,
or Black. This set has two uniformly charging devices (charging
apparatuses) that are a corona charging device, image exposing
device (exposing apparatus), and an image developing device
(developing apparatus). Unlike in the case of one-photo
conductor/four-rotations type process or laterally arranged four
photoconductors type process, without transferring the image formed
on a photo conductor previously to the recording medium or the
intermediate transfer member image forming processes, that is
uniformly charging, exposing, and developing, are performed for the
image kept on the photo conductor, and then the image of four
superimposed colors is formed at the identical position on the
photo conductor. Although the two charging devices do not always
have to be set up, one of them is used as a neutralization device
for removing toner electric potential that has an influence on
forming a latent image.
[0010] In the above apparatus, ozone natured harmful is generated
from the ten corona charging devices combined in the above set of
two charging devices, a charging device for the pre-transfer step,
and a charging device for the transfer step. So, airflow by fan is
provided to force the ozone into a filter that absorbs it.
[0011] Heretofore various approaches have been suggested concerning
the above image processing apparatus by the
one-photo-conductor/one-rotation-superimpose-type process. However,
each suggested apparatus includes one charging device or two
charging devices per color, as discussed previously. Moreover, a
non-contact type charging device, for example corotron or scorotron
type corona discharging device, is used so as to avoid disturbing
the image formed on the photo conductor before by contacting of the
charging device.
[0012] In the field of conventional image forming apparatuses, the
following documents are known. Japanese Laid Open Publication No.
2003-202752 discloses a developing device that transports toner
with a phase-shift electric field for developing, and Japanese
Patent No. 3385008 discloses an example of a charging device that
charges with a scorotron charging device.
[0013] In addition, an image forming apparatus that forms
mono-color images by using two different colors individually, but
forms full-color images formed superimposing color on color, is
known. The apparatus performs the following steps: charging
uniformly one time; forming a latent image with three different
potential levels by exposing images; developing the intermediate
potential pixel as a blank portion, the high potential pixel in
normal development method using black toner, for example, and the
low potential pixel in reverse development using red toner, for
example.
[0014] Further, the devices disclosed in Japanese Patent No.
3073126 and Japanese Patent No. 3170901 are known as an image
forming apparatus for forming a mono color image using three or
four colors individually by more than once exposing the image. And
the type of these apparatuses is called
one-photo-conductor/one-rotation multi-color type process image
forming apparatuses.
[0015] Moreover, it is known that a corona charging device, which
is used in the one-photo-conductor/one-rotation-superimpose-type
process, for example, generates much unnecessary ozone natured
harmful with required corona ion. Therefore conventional image
forming apparatuses are required to be equipped with a filter for
removing ozone. As a result, the control of airflow, using a fan,
for example, is also necessary. Consequently, a larger size and a
high production cost of the apparatus is a drawback. And there is
another problem that is the requirement of maintenance since a user
or a maintenance person needs to replace the filter at the end of
its lifetime.
[0016] On the other hand, contact type charging devices are known
for generating little ozone and thus have no need for the control
of airflow in the apparatus by fan, for example. However, using
contact type charging devices as a device applied to
one-photo-conductor/one-rotation-superimpose-type process causes
disturbance of the image formed on the conductor previously.
Therefore, contact type charging devices can not be in use
practically.
[0017] As discussed previously, in an image forming apparatus using
the one-photo-conductor/one-rotation-superimpose-type process,
quantities of the toner developed on the conductor previously are
supposed to vary since the photo conductor is charged uniformly
from one to three times for forming a latent image of the next
color. So a pre-transfer charge is required so as to get quantities
of each color toner lined up after forming a full color image using
four color toners before transfer. And again, the use of a
non-contact type discharging device is required so as to prevent
disturbance of the formed image on the conductor. In this regard,
the apparatus requires more space and cost, and frequent exchange
of the ozone filter because of its lifetime is shortened by the
pre-transfer charging with the non-contact type discharging
device.
[0018] In addition, in a case that the pre-transfer charge is
performed as described, allowing that the quantities of toner can
be recovered in average amount, it is impossible to reproduce with
accuracy including the distribution. Ordinary, the quantities of
electrostatic charge is adjusted optimally including the
distribution, not only for developing, but also for transferring,
so it would be the best to transfer while keeping the original
quantities of electrostatic charge including the distribution.
However, it is impossible to recover the original quantities.
Therefore a problem of losing image quality occurs also.
[0019] In the one-photo-conductor/one-rotation/multi-color-type
process described above, an apparatus forms toner image on a photo
conductor with multiple color toners by exposing and developing
repeatedly after charging uniformly once. After that, the apparatus
transfers the image onto the recording medium all at once. This
process is a method that form images of mono color in different
locations depending on each color. In other words, the process
cannot superimpose an image made of mono color toner on other image
made of other colors in the same location. Originally, the problems
to be solved and technical difficulties in the
one-conductor/one-rotation-multi-color-type process differs
entirely from those in the
one-photo-conductor/one-rotation-superimpose-type process.
Therefore, the technique of the multi-color type process can not be
applied to the technique of the superimpose-type process.
SUMMARY OF THE INVENTION
[0020] Accordingly to the foregoing description, an object of the
present invention is to provide an image forming apparatus and a
method to form full-color images using the
one-photo-conductor/one-rotation/one-charge-superimpose-type
process, which has the following advantages: easy construction,
lower cost, and easy maintenance.
[0021] According to the objects mentioned above, the present
invention provides an image forming apparatus comprising a charging
device configured to charge a moving image bearing member, a
writing device configured to form a latent image in a lower course
of said charging device by exposing a plurality of colors according
to image information, and a developing device configured to develop
the latent image with toner. Exposure and development is performed
more than once by the writing device and the developing device to
the image bearing member, which is charged once by the charging
device so that the color image is formed from superimposed toner of
each of the plurality of colors on the image bearing member.
[0022] Preferably, toner is not trapped by carriers. In addition,
preferably, a device to develop by making toner hopping with a
phase-shifting electric field EH developing device is used as the
developing device. Furthermore, preferably, the charging device to
charge the image bearing member is a contact-type developing
device, and the contact-type developing device is a charge
injection type developing device.
[0023] Further, it is possible to expose an image portion forming
pixel supposed to be adhered with toner at a next development so as
to decrease an electric potential of the image portion forming
pixel in absolute value to a lower level than an electric potential
of a blank portion forming pixel that is not exposed. In this case,
preferably, the exposing device is configured to expose to a blank
portion forming pixel that has a higher electric potential in
absolute value than another blank portion forming pixel of blank
portion pixels that are formed in a second or later exposure at a
same time as exposure to image portion is performed, so that an
electric portion of the blank portion forming pixel becomes a same
level as the electric potential of the another blank portion. Or
preferably, the exposing device is configured to expose to a blank
portion forming pixel that has a higher electric potential in
absolute value than another blank portion forming pixel of blank
portion pixels that are formed in the second or later exposure
before of after exposure to the image portion is performed so that
the electric portion of the blank portion forming pixel becomes a
same level as the electric potential of the other blank
portion.
[0024] In addition, it is possible to expose a blank portion
forming pixel supposed not to be adhered with toner at a next
development so as to decrease an electric potential of the image
portion forming pixel in absolute value to a lower level than an
electric potential of the blank portion forming pixel that is not
exposed. In this case, preferably the exposing device is configured
to expose a blank portion forming pixel supposed not to be adhered
with toner at a next development so as to decrease an electric
potential of the image portion forming pixel in absolute value to a
lower level than an electric potential of the blank portion forming
pixel that is not exposed. Or preferably, the exposing device is
configured to expose to an image portion forming pixel that has a
higher electric potential in absolute value than another image
portion forming pixel of image portion pixels that are formed in a
second or later exposure before or after exposure to the blank
portion is performed, so that electric portion of the image portion
forming pixel becomes a same level as the electric potential of the
another image portion.
[0025] Furthermore, preferably the charging device is configured to
charge in order that the electric potential becomes more than a
maximum electric potential of the layer of toner that is developed
on the image bearing member.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] While the specification concludes with claims particularly
pointing out and distinctly claiming the subject matter which is
regarded as the invention, the objects and features of the
invention and further objects, features and advantages thereof will
be better understood from the following description taken in
connection with the accompanying drawings in which:
[0027] FIG. 1 schematically shows an image forming apparatus
capable of forming color image of the present invention;
[0028] FIG. 2 shows a developing device of the image processing
apparatus;
[0029] FIG. 3 shows an enlarged view of an electrostatic
transporting roller;
[0030] FIG. 4 shows a driving waveform applied to the electrostatic
transporting roller;
[0031] FIG. 5 schematically shows a simulated time change of toner
position in EH development;
[0032] FIG. 6 schematically shows a simulated time change of toner
position after that shown in FIG. 5;
[0033] FIG. 7 schematically shows a simulated time change of toner
position after that shown in FIG. 6;
[0034] FIG. 8 schematically shows a simulated time change of toner
position after that shown in FIG. 7;
[0035] FIGS. 9A-9C schematically show forming an image of the first
color when the image forming apparatus forms a color image in a
superimpose-type process;
[0036] FIG. 10 shows forming an image of the second color when the
image forming apparatus forms a color image in a superimpose-type
process;
[0037] FIG. 11 shows forming an image of the third color when the
image forming apparatus forms a color image in a superimpose-type
process;
[0038] FIG. 12 shows forming an image of the fourth color when the
image forming apparatus forms a color image in a superimpose-type
process;
[0039] FIG. 13 shows development amount m/A per unit area of toner
for development potential difference in a conventional developing
method;
[0040] FIG. 14 shows development amount m/A per unit area of toner
for development potential difference in an EH developing
method;
[0041] FIG. 15 shows forming an image of the first color when
another image forming apparatus forms a color image in a
superimpose-type process;
[0042] FIG. 16 shows forming an image of the second color when the
image forming apparatus forms a color image in a superimpose-type
process;
[0043] FIG. 17 shows forming image of the third color when the
image forming apparatus forms a color image in a superimpose-type
process; and
[0044] FIG. 18 shows forming image of the fourth color when the
image forming apparatus forms a color image in a superimpose-type
process.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0045] Embodiments of the present invention are described in detail
with reference to the attached drawings. FIG. 1 and FIG. 2
schematically show an image forming apparatus configured to form a
full-color image. FIG. 1 schematically shows an image forming
apparatus capable of forming a color image in an embodiment of the
present invention, and FIG. 2 shows a developing device of the
image processing apparatus.
[0046] The image forming apparatus comprises a belt-shaped photo
sensitive conductor (OPC: organic photo conductor) as an image
bearing member 1, a contact-type charging roller 2 as a
contact-type charging apparatus (a charging device) used to charge
the image bearing member 1 uniformly, a developing apparatus 4Y
used to adhere yellow toner and develop a latent image that is
formed on the image bearing member by a writing apparatus 3Y, a
developing apparatus 4M used to adhere magenta toner and develop a
latent image that is formed on the image bearing member by a
writing apparatus 3M, a developing apparatus 4C used to adhere cyan
toner and develop a latent image that is formed on the image
bearing member by a writing apparatus 3C, a developing apparatus 4K
used to adhere black toner and develop a latent image that is
formed on the image bearing member by a writing apparatus 3K, a
transferring apparatus 5 used to transfer a full-color toner image
that is formed by superimposing each toner image on the image
bearing member 1, a fixing apparatus 6, and a sheet feeding
apparatus used to house transfer material 7. The four developing
apparatuses are located downstream of the charging roller 2 along
the rotating direction of the image bearing member 1 (indicated by
the arrow in FIG. 1) in order.
[0047] The image bearing member 1 is tensioned by means of a
transferring roller 11, a driven roller 12, an opposite
transferring roller 5B comprising a transferring apparatus 5,
opposite members 13Y, 13M, 13C, 13K that oppose the developing
apparatuses 4Y, 4M, 4C, 4K respectively, which rotate in the
direction indicated by arrow at a rate of 100 mm/sec, for example,
by rotation of the transferring roller 11. Note that each
developing apparatus will be referred to herein as the developing
apparatus 4 when they are not distinguished based on color.
[0048] The charging roller 2 is a contact type charging roller 16
mm in diameter, which is formed by laying rubbers 3 mm thick. The
value of resistance is adjusted by addition of carbon black.
[0049] Writing apparatuses 3Y, 3M, 3C, and 3K write the latent
image of each color sequentially to the image bearing member 1
charged uniformly once by the charging roller 2. Various sorts of
devices can serve as the writing apparatus, for example, light
scanning apparatus by laser, LED array, etc. Here, each writing
apparatus has one 5 mW laser diode, and exposes by modulating the
intensity of exposure according to each pixel (power
modulation).
[0050] The transferring apparatus 5 includes the transferring
roller 5A and the opposite transferring roller 5B. The fixing
apparatus 6 includes a heat roller 6A and a pressure roller 6B
located on the opposite side of the heat roller 6A. As the
transferring roller SA, a roller formed by covering a metal roller,
which has applied -500 V for transferring. A semi-conductive rubber
layer 3 mm thick can be used, for example.
[0051] In case the image forming apparatus serves as a copier,
image information loaded from a scanner (not shown) is exchanged to
write data treated with various sorts of image data processing, for
example, AID exchange, MTF correction, gray-scale processing, and
so on. In case the image forming apparatus serves as a printer,
such image information as page-description language or bit-mapped
image and so on, is exchanged to write data treated with various
sorts of image data processing.
[0052] Before forming of an image, the image bearing member 1
starts to rotate in the direction of the arrow in FIG. 1 in order
that the surface movement speed reaches a predetermined level.
Then, at the proper moment, the image bearing member is charged
uniformly once by the charging roller 2, and the writing apparatus
3Y exposes according to the write data of yellow by illuminating
laser beam 3a (referring to FIG. 2) on the charged image bearing
member 1. More specifically, changing electric potential in the
region of image portion by illuminating imposes the potential
difference from a non-image portion that is not illuminated. The
electrostatic latent image is produced from this potential
difference. Next, yellow toner is adhered on the image portion of
the electrostatic latent image by the developing apparatus 4Y. As a
result, a yellow toner image is formed on the image bearing member
1.
[0053] In the same way, the writing apparatus 3M exposes according
to the write data of magenta by illuminating laser beam 3a, after
which the electrostatic image for magenta is formed. Magenta toner
is adhered on the image portion of the electrostatic latent image
by the developing apparatus 4M. As a result, toner image is formed
on the image bearing member 1 by superimposing magenta toner image
on the yellow toner image. Furthermore, the toner image is formed
on the image bearing member 1 by superimposing cyan toner image on
the image made of yellow toner and magenta toner. Finally, the
toner image of full-color is formed on the image bearing member 1
by superimposing black toner image on these three toner image.
[0054] At the proper moment the transfer material 7 is fed from a
sheet feeder 8, and carried through a feed route 9. The toner image
formed on the image bearing member 1 is transferred to the transfer
material 7. Furthermore, the fixing apparatus 6 fixes the
transferred image, after which the transfer material 7 formed with
a full-color image thereon is ejected to a paper ejection part
10.
[0055] Referring to FIG. 2 also, the detail about the developing
apparatus 4 is described. FIG. 2 is an enlarged diagram showing the
developing apparatus in the image forming device.
[0056] The developing device 4 has an electrostatic transporting
member (electrostatic transporting roller) 42 that is roll-shaped,
a housing part 43 where toner is received, a supplying roller (a
developer bearing member) 44 to serve as a supplying device used to
supply the electrostatic transporting roller 42 with toner
particles in the housing part, and a recovery roller 45 used to
recovery the toner carried by the electrostatic transporting roller
42. The above device is contained in a case 41. The electrostatic
transporting roller operates to transfer powdered state toner by
means of the phase-shifted electric field for developing the
electrostatic latent image formed on the image bearing member
1.
[0057] The supplying roller (a developer bearing member) 44 is
arranged with a settled magnet inside. The developer in the housing
part 43 is supplied to the surface of the supplying roller 44 by
the rotation and magnetic attraction of supplying roller 44 and an
agitate screw 48. The thickness of developer on the supplying
roller 44 is restricted to a given quantity by a developer layer
thickness controlling device 46 placed opposite a circumference of
the supplying roller 44. The developer supplied by the supplying
roller 44 is carried to the region opposite the electrostatic
transporting roller 42 according to the rotation of the supplying
roller 44.
[0058] Here, the supplying roller 44 is applied with an electric
potential by means of a voltage applying device (not shown). And
the electrostatic transporting roller 42 is applied with an
electric potential for forming transporting electric field by means
of a voltage applying device (a driving circuit) that is described
later.
[0059] Accordingly, an electric field between the electrostatic
transporting roller 42 and the supplying roller 44 is created in
the region where the supplying roller 44 faces the electrostatic
transporting roller 42. Receiving the electrostatic force from this
electric field, the negatively charged toner dissociates from the
carrier, and moves toward the surface of the electrostatic
transporting roller 42. The toner that has reached the surface of
the electrostatic transporting roller 42 is transported to the
surface of the electrostatic transporting roller 42 by means of the
transporting electric field (phase shift electric field) formed by
the voltage applied to the electrode. In the present invention, the
method of supplying charged toner to the electrostatic transporting
roller 42 is not limited to the above-noted bi-component type
development. Alternative methods of development are available; for
example, mono-component type, charge-injection type, and supplying
from stored toners that are charged, etc.
[0060] During image processing, the electrostatic transporting
roller 42, which has a plurality of electrodes used to form the
electric field for transferring, developing, and recovering toner,
is placed opposite the image bearing member 1 in a non-contacting
state with a nearest distance of 50-1000 micrometers, optimally
150-400 micrometers. In this embodiment, the distance is 300
micrometers.
[0061] FIG. 3 is an enlarged diagram showing the surface facing the
image bearing member 1 of the above-noted electrostatic
transporting roller 42. The electrostatic transporting roller 42
includes a plurality of electrodes 102, which are arranged on a
support substrate 101 at intervals along the direction of
transporting toner in sets of n. The top of each electrode is
laminated with a surface protection layer 103 mode from inorganic
or organic insulating material. The surface protection layer 103
serves as an insulating electrostatic transporting surface forming
part that forms an electrostatic transporting surface 103a, and
also as a protection layer covering the surface of each electrode
102. In this embodiment, intervals between each electrode are 60
micrometers, and the width of electrode is 30 micrometers.
[0062] As the above-noted support substrate 101, the following
sorts of substrate can be used: a substrate made from an insulating
substrate, for example, resin substrate or ceramic substrate; a
substrate structured from substrate made from material having
conducting properties, for example Steal USE Stainless (SUS), that
is covered with insulting film, for example SiO.sub.2; and a
substrate structured from flexible material, for example, polyimide
film. The electrode 102 is formed by forming a conductive material
film 0.1-10 micrometers thick, optimally 0.5-2.0 micrometers thick,
and then developing a desired pattern of electrodes, for example,
using a photolithographic technique. For example, Ni--Cr can be
used as a conductive material. The surface protection layer 103 is
formed by forming film 0.5-10 micrometers thick, optimally 0.5-3
micrometers thick. For example, SiO.sub.2, TiO.sub.2, TiO.sub.4,
SiON, BN, TiN, or Ta.sub.2O.sub.5, can be used as the material of
protection layer 103.
[0063] In FIG. 3, lines leading out of the electrode 102 indicate
conducting wires used to apply voltage to each electrode 102. And
sites marked by a black circle of crossover sites indicate places
connected electrically, and other sites indicate an insulation
state. A driving circuit (a voltage applying device) 104 of the
main frame is configured to apply n-phased different driving
voltages to each electrode 102. In this embodiment, three phased
driving voltage is applied (m=3). However, any natural number
m>2 may have applicability on the condition that toners are
carried properly.
[0064] In this embodiment, each electrode 102 is connected to any
of contact points S11, S12, S13, S21, S22, or S23 of the developing
apparatus 4. S11, S12, S13, S21, S22, and S23 are connected
respectively to the voltage applying device 104, which gives
driving waveforms V11, V12, V13, V21, V22, and V23, of the main
frame in the condition that the developing apparatus 4 is loaded on
the image processing apparatus.
[0065] The electrostatic transporting roller 42 carries toner to
the proximity of the image bearing member 1. The electrostatic
transporting roller 42 is divided into a development region used to
form the toner image by adhering toner to the latent image on the
image bearing member 1, and a transporting region used to recovery
toner that is transported to the transporting region without being
used for development through the development region.
[0066] The development region exists only in the adjacent region to
the image bearing member 1, and the transporting region exists in
the whole area on the electrostatic transporting roller 42 except
for the development region. In this embodiment, a region where
toner is available to move by phase-shift electric field is
referred to as an "electrostatic transporting surface". In this
embodiment, the whole surface of the electrostatic transporting
roller 42 is an electrostatic transporting surface.
[0067] In the transfer region, driving waveforms V11, V12, and V13
are applied by the voltage applying device 104. In the development
region, driving waveforms V21, V22, and V23 are applied by the
electrodes 102.
[0068] In the following, the principle of electrostatic
transporting of toner with the electrostatic transporting roller 42
is described. Applying n-phased driving waveforms to a plurality of
electrodes 102 of electrostatic transporting roller 42 generates
the phase shift electric field (traveling wave electric field) by
the plurality of electrodes 102. Charged toner on the electrostatic
transporting roller 42 moves in the direction of transfer while
receiving a repulsive force and/or an attractive force.
[0069] For example, referring to FIG. 4, three-phase voltage, phase
A (VA), phase B (VB), and phase C (VC) is applied as a rectangular
wave to three electrodes 102, respectively, wherein the timing of
the three-phase waveforms are shifted by 120 degrees. The
rectangular wave has peak-to-peak electric pressure of 160V
(Duty=50%) and a frequency of 3 kHz. The charged toner moves over
on the electrostatic transporting roller 42 while hopping in sync
with the traveling wave electric field. The average amount Vb of
the traveling wave electric pressure operates similarly to what is
called developing bias in the developing region. Phase A (VA),
phase B (VB), and phase C (VC) correspond to the above-noted
electric pressure V11, V12, V13, V21, V22, and V23.
[0070] At this point, the height of toner hopping reaches 200-300
micrometers, so when the electrostatic latent image exists at the
height of 300 micrometers from the electrostatic transporting
roller 42, hopping toner enter into the electrical field formed by
the latent image (image portion) of the image bearing member 1, and
travels toward the latent image, and then develop the latent image.
As described above, the hopping toner is separated from carriers
and are not trapped with carriers. By contrast, in a blank portion,
the latent image forms an electric field that generates a force to
thrust back toner. So the toner, which travels toward the blank
portion (non-image portion), makes a U-turn in mid-course without
reaching the image bearing member 1, and is finally recovered by
the recovery roller 45. As above, because development is performed
with toner that is hopping by electrostatic transporting, this type
of development is called Electrostatic Hopping development, or EH
development.
[0071] Referring to FIG. 5-FIG. 8, a process for this type of
development is described in more detail. These figures show the
pattern diagrams of the result of a simulation showing the
positions of a toner 60 over time in the space formed by the image
bearing member 1 and the electrostatic transporting roller 42.
[0072] On the OPC (the image bearing member 1), a negative latent
image of 600 dpi, wherein 1 dot is 42 micrometers, is formed.
Development space 63 is created over the negative latent image by
the negative latent image. In addition, if the latent image is
larger, the development space spreads to an upper area. On the
other hand, the electrostatic transporting roller 42 is arranged
with electrodes 102A-102L. The hopping toner 60, which is
transported by the electrostatic transporting roller 42, have some
distribution about particle diameters and charge quantities. In
these figures, this distribution is depicted as circles having
different sizes.
[0073] When the negative charged toner 60 reaches the space 63, the
negative charged toner 60 goes toward the image bearing member 1,
and develops one dot of the latent image, because the toner 60 is
received with force to orient the toner 60 to the image bearing
member 1 in the space 63. In fact, as time advances as shown from
FIG. 5 to FIG. 8, it is realized that a part of the toner that is
hopped by the transporting roller 42 reaches the development space
63, where it is developed. At the same time, in the portion except
for one dot latent image, i.e., blank portion (non-image portion)
of the image bearing member 1, it is realized that hopping toner 60
starts to make a U-turn in mid-course to the side of the
electrostatic transporting roller 42.
[0074] This phenomenon wherein hopping toner is drawn into the
latent image (image portion), and are repelled in the blank portion
was confirmed using a high-speed camera. In this manner, EH
development develops only a latest latent image even if it is a
minute image, without disturbance for latent image formed
previously in a blank portion of the latest image.
[0075] Next, referring to FIG. 9 to FIG. 12, a process of
full-color image forming by a
one-photo-conductor/one-rotation/one-charge-superimpose-type
process in the image processing apparatus is described. The surface
potential for original color in each process is explained using
actual measurement values, however some parts that are unable to be
measured are explained using simulation values.
[0076] At first, as shown in FIG. 9A, the image bearing member 1
(also referred to as an "OPC belt") is charged with electricity to
-320V uniformly by impressing -320V with the power supply (not
shown) to the contact-type charging roller 2 in a condition of
running at a constant speed at 100 mm/sec. For this case, charging
is performed in order that the electric potential is more than a
maximum electric potential of the layer of toner that is developed
on the OPC belt as the image bearing member. This operation enables
superimposing toner images of each color by exposure and
development of multiple times per one electrostatic charge.
[0077] Next, as shown in FIG. 9B, the image is exposed with
relative light intensity LI=0.12 by means of the writing apparatus
3Y, and then a yellow latent image is formed. Only the pixels that
should be developed with a yellow toner are exposed
selectively.
[0078] The following includes a description of only nine colors,
i.e., six colors of yellow, magenta, cyan, red, blue, and green, in
addition to white and two kinds of black, because an explanation of
all colors would be very complicated. Two kinds of black are
distinguished below. One black, which is formed by superimposing
yellow, magenta, and cyan toner, is described as 3C, and the other
black, which is formed with only black toner, is described as
K.
[0079] As shown in FIG. 9B, four kinds of pixels, white (W),
magenta (M), cyan (C), and blue (B), and pixel of K that are not
developed with a yellow toner, are not exposed. On the other hand,
yellow (Y), red (R), green (G), and 3C, which are developed with
yellow toner, are exposed. As a result, the potential differences
of exposed pixels are decreased 100V from -320V to -220V. In the
following explanation, terms, such as "decrease", "increase",
"high", "low", are described in absolute value terms.
[0080] And as shown in FIG. 9C, reverse development is performed
for the yellow latent image with yellow toner yt by applying a
rectangular wave of -290V+-80V to the electrostatic transporting
roller 42 of the developing apparatus 4Y for yellow. At this time,
temporal and spatial average potential difference Vb of electrodes
102, which corresponds to conventional development bias, of
electrostatic transporting roller 42 is -290V, and a non-image
portion (non-exposure) potential difference is -220V. Consequently,
negative charged toner, which are made hopping between the OPC belt
(image bearing member) 1 and the electrostatic transporting roller
42, goes to an image portion (exposure) pixel in the electrostatic
force generated by the electric field between the OPC belt (image
bearing member) 1 and the electrostatic transporting roller 42, and
adheres thereto.
[0081] Then, the potential difference Vty of the toner, which
adheres on the OPC (image bearing member), is -60V. In other words,
the electric potential of the developed part has been -280V. This
-280V is created by adding toner electric potential -60 to the
electric potential -220V after exposure. And then the potential of
the developed part (-280V) is higher than the potential difference
(-220V) after exposure.
[0082] And as shown in FIG. 10(a), a magenta latent image is formed
by exposure with the writing apparatus 3M. Original colors that
have magenta toner as a component are the following four colors, M,
R, B, and 3C. The potential difference of each color is different;
M and B are -320V, R and 3C are -280V. In addition, the portion of
M and B do not have a toner, but a layer of yellow toner yt already
exists in the position of R and 3C. Therefore in consideration of
this potential difference and transmission coefficient of a yellow
toner layer for 780 nm light, the light intensity of the laser beam
is adjusted and then the same potential difference after exposure
V1=-180V is obtained.
[0083] As shown in FIG. 10(b), development is performed with
magenta toner mt by applying -250V+-80V to the electrostatic
transporting roller 42 of the developing apparatus 4M for magenta.
Because of reverse development, as with yellow, magenta toner mt
adheres to portions of M, R, B, and 3C, whose potential difference
is -180V, which is 70V lower than the applied potential difference
-250V. The toner potential difference is 60V as above, and each
potential difference of these points after exposure becomes
-240V.
[0084] For this case, there are two kinds of the potential
difference of the blank portion, -320V, which corresponds to each
original color K, W, and C, and -280V, which corresponds to each
original color Y and G. But, as noted above, the differential
between these two kinds of potential difference does not cause
scumming from adherence of toner on the blank portion, because, in
EH development, hopping toner does not get to the blank portion.
Although the differential between blank portions is larger as the
third and fourth development is performed, the problem of scumming
does not come up for the same reason. Furthermore, in EH
development, there is not a problem that carriers disturb the
yellow toner image formed previously by rubbing since toners is not
trapped by carriers.
[0085] Next, as shown in FIG. 11 (a), a cyan latent image is formed
by the developing apparatus 4C for cyan. As with writing the
magenta latent image, the light intensity is adjusted in
consideration of the potential difference of pixels that should be
exposed and the pre-adhered toner layer. Next, the cyan latent
image is formed by exposure of a laser to make the potential
difference after exposure -140V.
[0086] And, as shown in FIG. 11(b), reverse development is
performed for the cyan latent image by applying -210V+-80V to the
electrostatic transporting roller 42 of the developing apparatus 4C
for cyan with cyan toner ct, same as with development of yellow or
magenta toner. The toner potential difference is -60V for each and
the potential difference of the developed portion becomes
-200V.
[0087] In addition, as shown in FIG. 12(a), a black latent image is
formed on the remaining blank portion for white (-320V) with the
developing apparatus 4K for black such that the potential
difference after exposure becomes -100V. All the same, it is
possible to write a black latent image and to add black toner to
the portion where color of Y, M, C, R, G, B, and 3C have formed
already.
[0088] As shown in FIG. 12(b), reverse development is performed for
the black latent image by applying -170V+-80V to the electrostatic
transporting roller 42 of the developing apparatus 4K for black
with black toner kt, and then a black toner image is formed on the
white blank portion.
[0089] In this way, a full-color print is formed by charging once,
exposure (forming a latent image), and development four times,
transferring a full-color toner image formed on the OPC belt 1 by
applying transfer potential difference -500V to the transferring
roller 5, and fixing by the fixing apparatus 6.
[0090] Reflection density of the above provided full-color print
was around 1.6 when it was measured in a Macbeth densitometer, and
it exceeded the target value of 1.4. There was no scumming, and the
reflection density of the blank was 0.06, equal to the paper. The
ozone concentration inside the image forming apparatus is almost
0.
[0091] In EH development, toner is made hopping, and transferred to
a close position to the latent image on the image bearing member by
electrostatic transporting. At that point, there can be two kinds
of electric fields corresponding to a portion of the latent image.
It is determined that each toner is attracted to a portion of the
image, or repelled from the portion of a blank portion based on the
electric field, and then development is performed. So the
development sensitivity of EH development is higher than one of the
conventional development method. The above advantages concerning
reflection density and ozone concentration come from this higher
development sensitivity of EH development.
[0092] Next, the relationship between a conventional bi-component
development method and EH development is explained. In the
bi-component development method (magnetic brush development), which
is a representative example of a conventional development method, a
development amount m/A per unit area of toner for development
potential difference is shown in FIG. 13, for example
("Electrophotography Principles and Optimization", author: Merlin
Scharfe, translator: Fuji Xerox Research Institute, publisher:
CORONA PUBLISHING CO., LTD.)
[0093] The image density required for normal printing is 1.4, and
the toner mass m/A per the unit area required to get the image
density is 0.5 mg/cm.sup.2. In other words, in conventional
magnetic brush development, 300V is required as the potential
difference for development, which is the differential between the
potential difference of an image and the development bias. This is
required for developing, i.e., for separating carriers from toner
and adhering the toner to the image portion of the OPC latent
image. In fact, a differential of the same volume is required for
separating toner, which adheres to a blank portion on the OPC for
some reason, from OPC and getting back to the magnetic brush. A
combined potential difference of 600V is required.
[0094] Thus, in image forming devices such as a normal printer or a
copier, development is performed by charging -700V to the image
bearing member, exposure to make the potential difference of the
image portion -100V, and applying -400V as a development bias.
[0095] Therefore, within the scope of the conventional development
method, if image forming was tried in the
one-photo-conductor/one-rotation/one-charge-superimpose-type
process, the charging device of the image bearing member is needed
to be larger than -1800V. In this case, the electric field
impressed to photo conductor becomes 3 times the normal value, and
it shortens the lifetime of the photo conductor greatly.
Triplication of thickness of the photo conductor makes the electric
field impressed to the photo conductor the same level as the normal
level. But in the case of a dual-layered OPC, which is a kind of
conventional OPC that has a charge generation layer under the
charge transport layer, as positive holes generated by light
transport, these diffuse broadly in the charge transport layer of
triple thickness. Consequently, a formed image is so blurred that
it is not acceptable for practical use.
[0096] As a practical matter, even in jumping development that is a
non-contact type development, almost the same potential difference
described above is necessary to separate off toner from carriers
and to separate off toner that adheres to a blank portion in the
reverse direction. In fact, some toner adheres to a blank portion
because toner is reciprocating intensively between carriers and the
image bearing member despite non-contact type development.
[0097] In contrast, in EH development, the development sensitivity
is high at all as shown in FIG. 14. FIG. 14 shows that the required
development potential difference to get m/A=0.5 mg/cm.sup.2 per
unit area is only 70V. In addition, because in EH development toner
does not contact with a blank portion, a strong electric field
required to recovery toner is unnecessary. All that is required is
an electric field for recovering hopping toner moderately. For this
purpose, in the above embodiment, 30V is spared, but 10V is
sufficient in practice. Even in the case of 10V, scumming does not
occur.
[0098] Therefore, although in the above embodiment the potential
difference for the initial charge is 320V, a charge of an even
lower potential difference is able to form a proper image in
practice.
[0099] In addition, though FIG. 14 shows a case in which the
average ratio charge q/m is -23 .mu.C/g, as q/m decreases, the
required development potential difference to get m/A=0.5
mg/cm.sup.2 per unit area decreases pro rata. In view of this, it
is possible to form a proper image with a lower potential
difference.
[0100] As thus described, the image forming apparatus comprises a
charging device configured to charge an image bearing member that
is moving, a writing device configured to form a latent image in a
lower course of said charging device by exposing for each color
according to image information, and a developing device configured
to develop the latent image with toner. Exposure and development is
performed more than once by the writing device and the developing
device to the image bearing member that is charged once by the
charging device so that color image is formed from superimposed
toner of each color on the image bearing member. This operation
enables the image forming apparatus to form a full-color image in a
one-photo-conductor/one-rotation/one-charge-superimpose-type
process, which has the following advantages: easy construction,
lower cost, and easy maintenance.
[0101] Further, toner is not trapped by carriers, so the toner
image developed previously is not rubbed and disturbed with
carriers so that the quality of the image can be improved. In
addition, an EH developing device is used as developing device. The
EH developing device is a device to develop by making toner hopping
with a phase-shifting electric field, so that a high quality image
without scumming can be formed in a
one-photo-conductor/one-rotation/one-charge-superimpose-type
process. Furthermore, the charging device to charge the image
bearing member is a contact type developing device, so the amount
of generated ozone can be reduced.
[0102] Next, as another embodiment of the invention, an example is
described that a one photo conductor/one-rotation/one charge
superimpose-type process is performed not in EH development, but in
a conventional development method, for example, a contact type
magnetic brush method or a non-contact jumping method. In the
conventional development, toner contacts with a blank portion also,
unlike in EH development.
[0103] In the above-mentioned embodiment, even the electric
potential of a blank portion is left uneven during four times
development, and then a clean image can be obtained for EH
development. On the contrary, in the above conventional development
method, if a potential difference exists in a blank portion, toner
that arrives at the blank portion gets tied up and then contributes
to scumming. So the electric potential of blank portion is required
to be uniform. In this embodiment, the electric potential of blank
portion is operated to be uniform using image exposure.
[0104] At first, as a charging apparatus, a scorotron charging
device that has two wires and 30 mm width is used instead of the
charging roller 2 in the above embodiment. Further, as a developing
apparatus, a bi-component soft-touch developing device is used
instead of the developing apparatus 4 that performs EH development.
The soft touch development is implemented in reducing an amount of
digging into a development surface using an extended magnet brush.
The same toner as in the above embodiment is used, and its charge
amount is the same also.
[0105] Referring now to FIGS. 15-18, image forming in another
embodiment is explained in changing a potential difference based on
observation and simulation, same as in the above-mentioned
embodiment.
[0106] At first, as shown in FIG. 15(a), the surface of OPC belt 1
is charged by the scorotron charging device, and then gains
-2400V.
[0107] Next, as shown in FIG. 15(b), a yellow latent image is
written in the charge surface by the writing apparatus 3Y for
yellow, and then a potential difference of image (exposure) portion
is -1800V. In fact, in this embodiment, an image is formed with a
reverse development method, that is, reducing the absolute value of
the potential difference of an image portion pixel to less than the
potential difference of a non-image portion pixel that forms a
blank portion by exposing an image portion that should have adhered
toner in a next development, and then adhering toner thereto.
[0108] Next, as shown in FIG. 15(c), reverse development is
performed by a soft touch bi-component develop device at developing
bias Vb=-2100V with yellow toner yt. The potential difference after
development is the same 60V as in the foregoing embodiment, so that
the potential difference of a portion adhered with toner becomes
-1860V. In addition, the size of toner in the explanation diagram
of the foregoing embodiment is almost the same as a depth of toner
potential difference. But drawing in the same way as in the
foregoing embodiment, toner becomes too small to view easily. So,
in diagrams of this embodiment, the size of toner is drawn in a
larger size properly.
[0109] As shown in FIG. 16(a), the potential difference is reduced
to -1260V by writing a magenta latent image with the writing
apparatus 3M for magenta. At this time, there are parts which
yellow toner yt has already adhered to (corresponding to original
color R or 3C) and parts without toner (corresponding to original
colors M or B). The laser light intensity is changed according to
each part, and then exposure is performed so that an identical
surface potential -1260V is gained.
[0110] In addition, a part left as -2400V (corresponding to
original K, W, C) is exposed by a weaker laser beam so that the
potential difference of the blank portion is adjusted into a
potential difference of -1860V of the part developed with yellow.
Writing of a magenta latent image and exposing for adjusting the
potential difference of the blank portion can be performed by an
identical laser simultaneously.
[0111] In addition, reverse development is performed with magenta
toner mt at a development bias of -1020V by a soft touch
bi-component developing device in the same way.
[0112] As shown in FIG. 17(a), a cyan latent image is written by
the writing apparatus 3C for cyan, and exposure for forming a
uniform electric potential in a blank portion is performed
simultaneously. In fact, laser light is illuminated with an
adjusting light intensity according to an electric potential before
exposure and toner adhering amount, so that the electric potential
of a blank portion becomes -1320V, and the electric potential of
the image portion becomes -720V.
[0113] As shown in FIG. 17(b), reverse development is performed
with cyan toner ct at a development bias of -1560V by a soft touch
bi-component developing device in the same way as described
above.
[0114] Finally, as shown in FIG. 18(a), a black latent image is
written by the writing apparatus 3K for black, and exposure for
forming a uniform electric potential in a blank portion is
performed simultaneously. Here writing to the blank portion for
white is described. However, it is possible to form black toner on
color toner images that have formed already (corresponding to
original colors Y, M, C, R, G, B, 3C) in piles. Laser light is
illuminated in the same way as above described for Y, M, or C, so
that the electric potential of blank portion becomes -780V and the
electric potential of image portion becomes -180V.
[0115] As shown in FIG. 18(b), reverse development is performed
with black toner kt at a development bias of -480V by a soft touch
bi-component developing device.
[0116] After that, the full-color image formed on the OPC belt is
transferred to the transfer material 7 by applying -1.5 kV to the
transferring roller, and the image is fixed through the fixing
apparatus 6.
[0117] In this way, same as in the above embodiment, full-color
image can be formed in one charge.
[0118] However, one of the formed images had a rubbed pattern, and
did not show high sharpness, because a toner image formed earlier,
for example, the yellow toner image is rubbed by magnetic brush of
the next magenta development even in soft touch development. In
addition, as printing is performed repeatedly, a color mixture of
toner is generated. For example, yellow toner comes to be mixed in
the developing apparatus for magenta. Moreover, for each color, the
number of spot images, which is said to be caused by electric
breakdown of the photosensitive conductor, increased gradually. A
spot image is a spot that is supposed to be formed as an image, but
remains not formed finally. Consequently, it is speculated that the
charge electric potential difference is beyond the permissible
level of the OPC 20 micrometers thickness. In addition, the ozone
density was considerably high, and did not make much difference
from the four-times-charge type process.
[0119] In other words, as in a conventional development method,
formation of a full-color image by the
one-photo-conductor/one-rotation/one-charge-color-superimpose-type
process is possible. However, it is not practical, as described
above, although it allows for slight miniaturization of the image
forming apparatus.
[0120] As described above, in the case of conventional development,
in exposure for illuminating to a blank portion a forming pixel
that has a higher electric potential in absolute value than another
blank potion forming pixel of blank portion pixels that are formed
in a second or later exposure is performed at the same time as
exposure for illuminating to an image portion is performed, so that
an electric portion of the blank portion forming pixel becomes the
same level as the an electric potential of the another blank
portion. This operation enables prevention of scumming, even in
conventional development in which toner also contacts with blank
portions.
[0121] In this case, it is possible also that exposure for
illuminating to a blank portion forming pixel that has a higher
electric potential in absolute value than another of blank portion
pixels that are formed in a second or later exposure is performed
before or after of exposure for illuminating to an image portion is
performed using another optical system of exposure, so that an
electric portion of the blank portion forming pixel becomes the
same level as the electric potential of the other blank portion.
This operation enables exposure to be performed sharply at a high
resolution for forming an image, and not sharply at a low
resolution for uniformizing a blank portion, by using different
exposure systems, respectively.
[0122] In addition, in the above embodiment, development is
performed using the reverse development method so that toner, which
is charged with the same polarity as the charge polarity of the
image bearing member, is adhered selectively to a portion charged
with a lower electric potential. However, it can be performed also
in normal development that toner, which is charged with a different
polarity as the charge polarity of the image bearing member, is
adhered selectively to a portion charged with a higher electric
potential. This normal development can be used in both EH
development and the conventional development in a similar
manner.
[0123] In other words, it is possible also that forming a toner
image of each color by exposing a pixel forming blank portion
supposed not to be adhered with toner at the next development, so
as to decrease the electric potential of the pixel in absolute
value to a lower level than the electric potential of the not
exposed portion forming image in the normal development method.
[0124] However, in the case of the conventional development method,
similarly to the case of forming a toner image in the reverse
development method, exposure for illuminating to a blank portion
forming pixel that has a higher electric potential in absolute
value than another blank portion forming pixel of blank portion
pixels that are formed in the second or later exposure is performed
at the same time as exposure for illuminating to image portion is
performed, so that electric portion of the blank portion forming
pixel becomes the same level as the electric potential of the other
blank portion. This operation enables prevention of scumming, even
in conventional development in which toner also contacts with blank
portions.
[0125] Alternatively, exposure for illuminating to a blank portion
forming pixel that has a higher electric potential in absolute
value than another blank portion forming pixel of blank portion
pixels that are formed in the second or later exposure is performed
before or after exposure for illuminating to an image portion is
performed using another optical system of exposure, so that an
electric portion of the blank portion forming pixel becomes the
same level as the electric potential of the other blank portion.
This operation enables exposure to be performed sharply at a high
resolution for forming an image, and not sharply at a low
resolution for uniformizing a blank portion, by using different
exposure systems, respectively.
[0126] While the present invention has been described with a
preferred embodiment, this description is not intended to limit the
scope of the invention. Various modifications of the embodiment
will be apparent to those skilled in the art. Such modifications or
embodiments also fall within the true scope of the present
invention.
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